This invention relates generally to improvements in methods and apparatus for the measurement of blood pressure and heart rate and, more particularly, to a new and improved electronic sphygmomanometer system enabling very rapid, accurate, reliable and easily obtained blood pressure and heart rate measurements.
It is common practice in the medical arts, as in hospitals and doctor's offices, to employ an auscultation technique for measuring the blood pressure of a patient by using the characteristics of the so-called korotkoff sounds to determine the systolic and diastolic values of the patient's blood pressure.
The kortkoff method typically makes use of an inflated cuff surrounding a portion of the patient's upper arm. Sufficient inflation of the cuff closes off or completely occludes the brachial artery of the patient. As air is released and the cuff is slowly deflated, a point is reached at which the occluded artery begins to open for a very brief period during each cardiac cycle. At this point, the cuff pressure, which is assumed in using this process as being approximately equal to the blood pressure in the brachial artery, will be that of the peak pressure obtained during the cardiac cycle, this pressure being known in the medical arts as the systolic blood pressure.
Detection of the point at which the artery first opens may be made by any suitable listening device such as a stethoscope or microphone applied to the arm over the artery, usually at the downstream end of the inflated cuff. As the artery opens, auscultatory sounds caused by the pulsating blood flow or tubulence in the blood stream below the occlusion are sensed as a sudden rush of blood by the listening device, and these sounds are referred to in the medical arts as the well known korotkoff sounds. At the point of first detection, where the decreasing cuff pressure is matched by the maximum blood pressure, medical personel skilled in the auscultation technique can detect the sudden blood flow in the artery and the onset of korotkoff sounds, and thereby determine the systolic blood pressure.
As the pressure in the cuff continues to drop, the korotkoff sounds continue substantially in synchronization with the blood pressure pulses produced during successive cardiac cycles. Eventually, however, a point is reached at which the artery remains open during the entire cardiac cycle and, at this point, the korotkoff sounds appear to cease entirely. The cuff pressure at this point approximates the lowest blood pressure reached during the cardiac cycle, with the heart essentially at rest, and this is known as the diastolic blood pressure.
Hence, it will be apparent that, if values of the decreasing cuff pressure are correlated with the korotkoff sound output of the stethoscope or microphone, the cuff pressure at the time the first korotkoff sound occurs is approximately equal to the systolic blood pressure, while the cuff pressure at the time the last korotkoff sound occurs is approximately equal to the diastolic blood pressure encountered during the measurement process.
It will be apparent from the foregoing that conventional blood pressure measurement procedures using an inflatable cuff and a suitable listening device are prone to a number of significant deficiencies. In this regard, medical personnel making such measurements are required to make rather difficult and sometimes highly subjective determinations regarding the presence or absence of korotkoff sounds which may be of relatively low and difficult to detect amplitudes and are often intermixed and easily confused with ambiguous signals generated by artifacts and both internal and external noise. Such artifacts and noise may be due to cardiovascular irregularities and even to the inadvertent bumping of the blood pressure cuff or the movement of the patient while the blood pressure is being measured. Moreover, noise and artifact signals generally appear to be produced more frequently in sick patients than in healthy patients, so that the process is oftentimes more difficult to perform accurately in those instances where the very requirement for a high degree of accuracy is greatest. In addition, the determination of the end points for the onset and cessation of the korotkoff sound pulse train is somewhat uncertain and, therefore, subject to further inaccuracy in the absence of considerable training and much experience on the part of skilled medical personnel.
Accordingly, blood pressure measurements by the conventional sphygmomanometer are subject to a number of human measurement errors such as bias from past medical records, poor hearing, poor operator technique, distractions, improper operator training, missing an auscultatory gap, confusion by artifacts as may be due to bigeminal pulse biqniny, arrhythmias, and other cardiovascular irregularities, misinterpretation of the diastolic pressures, particularly for those cases where the fourth and fifth phases of the korotkoff sounds are indistinct and, as previously noted, misinterpretation of the presence or absence of a heart beat where the heart beats may be weak. The need to eliminate these human measurement errors, as well as the need to increase the speed and decrease the difficulty of blood pressure measurement has resulted in a continuing search for new and improved methods for the automatic non-invasive measurement of blood pressure.
Since there are relatively few persons possessing the requisite high levels of skill and experience to obtain consistently accurate blood pressure measurements using conventional manual auscultation techniques, various attempts have been made in the prior art to eliminate the aforedescribed deficiencies by mechanizing and automating the measurement process, so that the subjective factors introduced when an untrained person attempts to measure blood pressures can be eliminated and, further, to provide some discrimination against artifacts and noise. However, such automatic systems for measuring blood pressure and, typically, associated heart rate, have generally proven to be overly sensitive to spurious signals generated by artifacts and noise and have proven, therefore, to be in many instances less accurate than medical personnel using tried and true manual procedures. As a consequence, automatic korotkoff sound monitoring systems for determining blood pressure by the auscultation method have experienced only limited acceptance by the medical profession.
A number of techniques, other than auscultatory, have also been employed in the prior art to measure blood pressure, such as volumetric methods, Doppler or impedance techniques, and oscillometric techniques wherein the blood pressure waveform is analyzed. In the latter oscillometric method, the fluctuations in cuff pressure are used to measure blood pressure. Through comparison with other methods of blood pressure measurement, it has been determined that a correlation exits between the relative amplitudes of the blood pressure oscillations and the systolic, diastolic and mean pressures. The mean pressures are said to occur at the point where the pressure oscillations are at a maximum, the systolic and diastolic where they are at some percentage, typically about one-half, of the peak amplitude. However, these points can only be considered approximations, at best, and can vary considerably from individual to individual.
Hence, notwithstanding the recognition of the need for methods and apparatus which would be capable of automatically or semi-automatically measuring blood pressure, and the effort which has theretofore been exerted in attempts to fulfill that need, a reliable and effective method, which can be implemented in an instrument of reasonable cost, has not been provided by the prior art.
In recent years, efforts by the inventor of the present invention have provided substantial improvements in automated sphygmomanometer systems and have relied upon the detection of specified korotkoff sound precursors to certify genuine korotkoff sound signals and reject artifacts and noise. Such a blood pressure measuring system is described in co-pending application Ser. No. 845,081, filed Oct. 25, 1977, entitled "Electronic Sphygmomanometer", inventor Heinz W. Georgi, now U.S. Pat. No. 4,313,445, issued Feb. 2, 1982. Efforts by the inventor have continued to further refine and enhance the basic system and techniques set forth in the aforementioned application and to thereby substantially improve the accuracy, reliability and practicality of the blood pressure and related measurements obtainable.
Hence, those concerned with the development and use of automatic sphygmomanometers in the medical field have long recognized the continuing need for further improvements in sphygmomanometer systems which would enable more accurate and reliable blood pressure and heart rate measurements to be made and which would obviate the need for a high degree of skill and subjective expertise on the part of medical personnel making such measurements. The present invention clearly fulfills these needs.